This application is based on, and claims priority to, Japanese application number 9-246901, filed on Sep. 11, 1997, in Japan, and which is incorporated herein by reference.
This application also claims priority to Japanese application 9-224056, filed on Aug. 20, 1997, in Japan, and which is incorporated herein by reference.
This application is related to U.S. application titled xe2x80x9cMETHOD AND APPARATUS FOR MINIMIZING THE INTENSITY OF A SPECIFIC FREQUENCY COMPONENT OF AN OPTICAL SIGNAL TRAVELLING THROUGH AN OPTICAL FIBER TRANSMISSION LINE TO THEREBY MINIMIZE THE TOTAL DISPERSIONxe2x80x9d, filed on Mar. 19, 1998, in the United States, and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method and apparatus for optimizing dispersion in an optical fiber transmission line in accordance with the power level of an optical signal travelling through the optical fiber transmission line. More specifically, the present invention relates to a method and apparatus which sets the dispersion at a specific point along the optical fiber transmission line to zero, and then adds dispersion downstream of the specific point to optimize the total dispersion in accordance with the power level of the optical signal.
2. Description of the Related Art
Optical transmission systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. For example, optical transmission systems at 10 Gb/s are now in practical implementation in trunk-line optical communications. However, as users require larger amounts of information to be rapidly transmitted, a further increase in the capacity of optical transmission systems is required.
Time-division multiplexing (TDM) (including optical time-division multiplexing (OTDM)) and wavelength-division multiplexing (WDM) are being considered as candidates for such high capacity optical transmission systems. For example, with regard to TDM techniques, a significant amount of worldwide research is being performed on 40 Gb/s systems.
Chromatic dispersion (group-velocity dispersion (GVD)) is one of the factors limiting the transmission distance in a 40 Gb/s system. Since dispersion tolerance is inversely proportional to the square of the bit rate, the dispersion tolerance, which is about 800 ps/nm at 10 Gb/s, is reduced by a factor of 16 to about 50 ps/nm at 40 Gb/s.
For example, in measured experiments, an optical time-division multiplexed (OTDM) signal with a signal light wavelength of 1.55 xcexcm (where transmission loss in silica fiber is the lowest) was transmitted over a distance of 50 km through a single-mode fiber (SMF). The SMF had a zero dispersion wavelength of 1.3 xcexcm. This type of SMF is the type of fiber most widely installed around the world. The input signal light power was +3 dBm, and the bit rate was 40 Gb/s. Dispersion compensation was performed using a dispersion-compensating fiber (DCF). The width of the dispersion compensation value range allowed in order to hold the power penalty (degradation of optical signal reception sensitivity through transmission) to within 1 dB (dispersion compensation tolerance) was 30 ps/nm. This value translates to length of 2 km or less of an SMF with a chromatic dispersion value of 18.6 ps/nm/km.
Furthermore, in a land system, repeater spacing is not uniform. Thus, very precise dispersion compensation must be performed for each repeater section.
On the other hand, dispersion in a transmission line changes with time due to changes, for example, in temperature. For example, in the case of an SMF 50 km transmission, when the temperature changes between xe2x88x9250 to 100xc2x0 C., the amount of change of the transmission line dispersion is estimated to be as follows:
(Temperature dependence of zero dispersion wavelength of transmission line)xc3x97(Temperature change)xc3x97(Dispersion slope)xc3x97(Transmission distance)=0.03 nm/xc2x0 C.xc3x97150xc2x0 C.xc3x970.07 ps/nm2/kmxc3x9750 km=16 ps/nm.
This value is greater than one half of the dispersion tolerance of 30 ps/nm and cannot be overlooked when designing the system.
In the above-described measured experiments, when the amount of dispersion compensation was optimized at xe2x88x9250xc2x0 C. at the start of system operation, if the temperature subsequently rose to 100xc2x0 C. during system operation, the criterion of a 1 dB penalty could not be satisfied (worst case condition).
Further, depending on the characteristic and construction of a dispersion compensator, the amount of dispersion compensation can only be set discretely, sometimes leaving no alternative but to set the dispersion compensation amount to a value slightly displaced from an optimum value at the start of system operation. In that case, there arises a possibility that the criterion of 1 dB penalty may not be satisfied even when the temperature change is smaller than 150xc2x0 C.
Accordingly, it is an object of the present invention to provide an automatic dispersion compensation system for use in ultra high-speed optical transmission systems at 40 Gb/s or higher rates.
It is a further object of the present invention to provide such an automatic dispersion compensation system so as to optimize the amount of dispersion compensation for each repeater section of the optical transmission system at the start of system operation, and also to perform optimization of the dispersion compensation amount according to the changing dispersion value of the transmission line during system operation.
It is an additional object of the present invention to provide such an automatic dispersion compensation system for SMF transmission systems, but that can also be used in other systems that use 1.55 xcexcm band dispersion-shifted fiber (DSF) having a low chromatic dispersion value at that wavelength.
Moreover, it is an object of the present invention to provide a dispersion control method and apparatus that can properly control chromatic dispersion in an optical transmission line even when signal light power is so large that nonlinear effect becomes pronounced.
Objects of the present invention are achieved by providing a method and apparatus which (a) determines an optimum amount of total dispersion of an optical transmission line corresponding to a power level of an optical signal transmitted through the optical transmission line; (b) controls dispersion of the optical transmission line so that the total dispersion up to a specific point along the optical transmission line becomes approximately zero; and (c) adds dispersion to the optical transmission line to obtain the determined optimum amount of total dispersion. When adding dispersion, the dispersion can be added to the optical transmission line at a point which is downstream of the specific point.
The process of controlling dispersion in (b), above, can be performed in several different manners. The example, the process of controlling dispersion can include (i) detecting the intensity of a specific frequency component of the optical signal, the optical signal having an intensity v. total dispersion characteristic curve with at least two peaks; and (ii) controlling the amount of total dispersion of the transmission line to substantially minimize the intensity of the specific frequency component between the two highest peaks of the intensity v. total dispersion characteristic curve of the optical signal. Assuming that the optical signal is modulated by a data signal having a bit rate of B bits/second, then the specific frequency component is preferably a B hertz component of the optical signal.
Alternatively, the process of controlling dispersion in (b), above, can include (i) detecting the intensity of a specific frequency component of the optical signal, the optical signal having an intensity v. total dispersion characteristic curve with at least two peaks; and (ii) controlling the amount of total dispersion of the transmission line so that the intensity of the specific frequency component is at a midpoint between the two highest peaks of the intensity v. total dispersion characteristic curve of the optical signal.
Further, the process of controlling dispersion in (b), above, can include (i) detecting the intensity of a specific frequency component of the optical signal, the optical signal having an intensity v. total dispersion characteristic curve with a corresponding eye opening; and (ii) controlling the amount of total dispersion of the optical transmission line to substantially minimize the intensity of the specific frequency component in the eye opening.
Moreover, the process of controlling dispersion in (b), above, can include (i) detecting the intensity of a specific frequency component of the optical signal, the optical signal having an intensity v. total dispersion characteristic curve with a corresponding, allowable eye opening range; and (ii) controlling the amount of total dispersion of the transmission line to maintain the intensity of the specific frequency component along a point on the intensity v. total dispersion characteristic curve which is within the eye opening.
Objects of the present invention are also achieved by providing a method and apparatus of controlling dispersion in an optical transmission line having an optical signal transmitted therethrough. The method and apparatus (a) set the power of the optical signal to a first value producing substantially no nonlinear effect in the optical transmission line; (b) control the amount of total dispersion of the optical transmission line to be approximately zero; (c) change the power of the optical signal to a second value different from the first value; (d) determine an optimum amount of total dispersion of the optical transmission line corresponding to the power of the optical signal being at the second value; and (e) add dispersion to the optical transmission line to obtain the determined optimum amount of dispersion.
Objects of the present invention are further achieved by providing a method and apparatus of controlling dispersion in an optical transmission line having an optical signal transmitted therethrough. The method and apparatus (a) set the power of the optical signal to a value producing substantially no nonlinear effect in the optical transmission line; (b) control the amount of total dispersion of the optical transmission line to be approximately zero while the power is set to said value; (c) increase the power of the optical signal; and (d) add dispersion to the optical transmission line to obtain an optimum amount of dispersion corresponding to the increased power of the optical signal. The process of controlling the amount of total dispersion can take the form of any of the processes described above for controlling dispersion.